1. Field of the Invention
The present invention relates to an insulating adhesive tape utilized as material in electronic industries, the tape having high electrical insulation and heat-resistance property and capable of being bonded thermo-melting and, lead frames and semiconductor devices employing the tape.
2. Description of Prior Art
In the semiconductor device field, with sophisticated functions, high speed and high integration of LSIs, the IC chips tend to grow in size and an amount of heat generated from the same is increasing.
The specified packages cannot be flexibly changed in size, conversely, it is requested to miniaturize even if IC chips are designed to be large. Hence the gap between the IC chip and the inner wall surface of the IC package tends to be small. There have been problems as for the strength and reliability of the lead frame buried in the package.
In order to overcome such problems, a technology where a lead frame is arranged on the lower or upper portion of an IC chip is proposed and put in practical use, as described in Japanese Patent Application Laid-open No. SHO-61-218139 (1986) and U.S. Pat. No. 4,862,245.
In order to cope with heat increasingly generated from an IC chip, a technology has been proposed and used practically in which a heat spreader with good thermal conductivity and large thermal capacity is proposed and used in place of a chip mounting die or tub and the heat spreader is fixed to inner leads by using an insulating material.
In this device, an adhesive insulating tape is used to prevent the lead frame from being in direct contact with an IC chip or a heat spreader.
Conventionally such an adhesive insulating tape is formed by coating an adhesive resin such as epoxy resin, acrylic resin on a base supporting film (insulating substrate) such as polyimide base film. The insulating tape adheres a lead frame to an IC chip or a heat spreader.
This method has a disadvantage in that ionic impurities, contained in epoxy resin, acrylic resin or the like are eluted by using for a long time, thus destroying a circuit formed on the IC chip so that the reliability of semiconductor devices is degraded.
Moreover, when a short time bonding is executed at high temperatures to improve the productivity, a chemical component of the adhesive agent is partially volatilized and contaminates the surface of the lead frame, IC chip and heat spreader, thus decreasing the affinity to half-melted metal on the surface thereof to thus injure the reliability of the wire bonds electrically connecting an IC chip to a lead frame.
In order to avoid the above problem, curing an adhesive agent at low temperature takes a long time, thus resulting in insufficient level of productivity from industrial view point.
In order to overcome those disadvantages, Japanese Patent Application Laid-open Nos. HEI-2-15663 (1990) and HEI-2-36542 (1990) propose that an amide series or imide series adhesive agent is used for a tape.
However, according to the technology disclosed in the Japanese Patent Application Laid-open No. HEI-2-15663 (1990), after a semicuring polyimide series agent adheres a tape to a lead frame, it is necessary to remove the solvent and to complete the imidization reaction. In this case, since the produced solvent and water contaminate a lead frame, a contaminant removing step is needed , thus leading to an insufficient productivity.
In the adhesive agents for an adhesive insulating tape according to the technology taught by the Japanese Patent Application Laid-open No. HEI-2-36542 (1990), a polyether amide or polyether amideimide being a thermoplastic polymer is used on one surface of a tape adhered with a lead frame while a thermosetting polyimide die bonding agent or epoxy die bonding agent is used on the other surface of the tape adhered with an IC chip.
U.S. Pat. No. 4,862,245 discloses generally a concept in which an adhesive layer is formed on both sides of an alpha barriers such as polyimide film to fix a semiconductor chip and a lead frame, and the adhesive layer being formed by the material selected from epoxy, acryl, silicon, polyimide, arid polyimide containing silicon. It is described that an adhesive layer selected form the group consisting of acryl and epoxy is particularly preferable for the second adhesive layer on the lead frame. Although the prior art publication describes polyimide alone but does not describe the sorts of polyimide adhesive agents and merits of the polyimide. It is not clear that what sort and molecular structure of adhesive agent is optimum.
U.S. Pat. No. 4,543,295 proposes an adhesive polyimide film totally different from the above mentioned adhesive insulating tapes. The film, for example, is formed by coating a thermoplastic polyimide on both surfaces of a polyimide film and may be used to adhere metal and polyimide film.
This tape can adhere a lead frame, a heat spreader for semiconductor device, and an IC chip with a polyimide buffer coat. This tape may possibly improve the disadvantages to be removed of the foregoing adhesive insulating tape: ionic impurities, volatile component and water absorption generated at adhering time.
This specification describes that all sorts of thermoplastic polyimide can be used. However, the exemplified polyimide are only LARC-2, LARC-3 and LARC-4 respectively having a glass transition temperature of 247.degree. C., 269.degree. C. and 297.degree. C. Furthermore, those adhesive agents are relatively brittle. In the adhering condition, since it is stated that the temperature ranges 340.degree. C. to 370.degree. C. and the pressure ranges 3 to 21 kg/cm.sup.2 and suitable time is 5 minutes, it takes disadvantageously long time for adhering.
Remarkable advance of current semiconductor related technology and very high productivity to be required have boosted the requirement level in productivity and physical property such as adhesive strength. For example, it is required that the adhesion time (time required for adhesion) is usually not more than 10 seconds, preferably not more than 3 seconds.
In some cases, a semiconductor chip is not covered with polyimide, but is covered only with a ceramic such as silicone nitride or glass such as phosphoric glass.
Under such conditions, the adhesive strength of LARC-2, LARC-3 and LARC-4 to semiconductor material are not enough.
For example, an adhesive tape made from LARC-2 was adhered to 42 NiFe alloy sheet being a lead frame material for semiconductor devices, under the conditions that the temperature is 370.degree. C., the pressure is 21 kg/cm.sup.2, and the heating adhering time is 5 seconds. The peeling strength of the resultant tape was only 0.5 kg/cm. By extending the thermo-bonding time to 5 minutes, a peeling strength of 1.5 kg/cm was obtained.
It is needless to say that the adhesive strength between a base supporting film and an adhesive agent is important for the adhesive insulating films. However, the thermoplastic polyimide adhesive agent has not been realized that can highly maintain industrially and stably the adhesive strength to both sides of an insulating substrate. Particularly, in the case of the insulating substrate with an imide bond, it seems impossible to obtain a stable, high adhesive strength through corona treatment. A suitable surface treatment has not been found because with the sandblast surface roughing, minute sands remain in the surface to cause impurity ions or a rays. In recent years, Japanese Patent Application Laid-open No. HEI-5-59344 (1993) discloses generally a method for treating physically and chemically the surface of an insulating substrate to be coated and then adhering a certain of thermoplastic polyimide adhesive agent. However, there is no description regarding that what method is optimum to use. Japanese Patent Application Laid-open No. HEI-4-74372 (1992) describes that plasma processing a film such as polyimide film to modify a surface improves the adhesive strength to the adhesive agent. However, only thermosetting acrylic series adhesive agent and thermosetting polyamide adhesive agent are illustrated as adhesive agent. There are no explanations on other sorts of adhesive agents.
The present invention was performed in order to overcome the above mentioned problems. An object of the present invention is to provide an insulating adhesive tape useful for improving the productively and performance of semiconductor devices. Another object of the present invention is to provide a lead frame and a semiconductor device employing the above described tape.
More specifically, it is an object to provide an insulating adhesive tape with good characteristics, reliability, and industrial productivity, which is applicable for bonding a semiconductor to a metal conductor such as lead frame, heat spreader.
Still another object of the present invention is to provide an insulating adhesive tape employing a thermoplastic polyimide which has much better properties than the thermoplastic polyimide disclosed in the U.S. Pat. No. 4,543,295, has a higher adhesive strength to semiconductor material in shorter time, in comparison with the above prior art, and being able to be supplied at low price and in mass production base.
The insulating adhesive tape can maintain its excellent insulating property for a long service period of time. The insulating adhesive tape also has an adhesive temperature (the temperature at which adhesion is carried out) of not more than 400.degree. C., a short adhesion time not more than 10 seconds, and an adhesive strength enough to various semiconductor materials. In adhesion, the insulating adhesive tape also does not occur contamination injurious to the wire-bonding on the surfaces of lead frames and IC chips. The insulating adhesive tape has enough heat resistance, to withstand heating during a wire bonding step so that it does not allow a lead to make a slightest movement, thereby making wire bonding strength high enough to obtain excellent electronic reliability.
Further another object of the present invention is to provide a lead frame and a semiconductor device manufactured using the insulating adhesive tape mentioned above.